In a supplemental inflatable restraint (SIR) system a deployment circuit normally includes a microprocessor and accelerometer for sensing when a crash occurs and actuating a firing circuit which inflates an air bag. The firing circuit includes a squib which is fired upon command of the microprocessor. Both the microprocessor and the firing circuit require an electrical energy source which is dependable during a crash. The SIR system is normally supplied by the vehicle ignition voltage which depends on the battery and in some cases the battery could become disconnected during a crash. Thus it is already known to provide an energy reserve which is sufficient to supply both the deployment circuit and the firing circuit for reliable operation during a crash.
A known system for providing an energy reserve comprises capacitors connected to both the deployment circuit and the firing circuit and a charging circuit for supplying energy from the ignition circuit to the capacitors. The capacitor voltage is preferably somewhat higher than normal ignition voltage to minimize the size of capacitors required for a given energy reserve. Accordingly, high current type switching regulators have been used for charging the capacitors and maintaining the operation of the deployment circuit. The high current source is required since the microprocessor and accelerometer normally draw about 40-50 mA of load current for continuous operation in addition to that needed to charge the energy reserve capacitors. It is, however, desirable to avoid the high expense of the switching regulators.